If you can get hold of the old "armour wash" that might work, I think it's a bit metallic, otherwise a standard black wash from the new
GW range should work on metals? You could also try a purple wash if you wanted. That might compliment the rest of the purple in the model?
I like the idea for the scarabs though, bases with a bit of something going on are always fun to look at on the board. Just needs flock or sand to finish it, maybe some wash and highlights too.
Looks like a good start to, I'm sure there will be some Necron paint vids on YouTube too for extra help, I find these helpful.